This is a rather odd error to get as the file:
/includes/modules/downloads.php

calls the file:
/includes/templates/template_default/templates/tpl_modules_downloads.php

Could you check both files on the server to ensure that they are the current versions?

Also check if you have an override file for:
/includes/modules/downloads.php

which would be in your templates and overrides directory:
/includes/modules/your_template_dir/downloads.php

also check the file:
/includes/templates/template_default/templates/tpl_account_history_info_default.php

which calls the file:
/includes/templates/template_default/templates/tpl_modules_downloads.php


Then test your site and see if the error is generated when you go to the Account History ... or just from the checkout_success page ...